1. Field of the Invention
The present invention relates to an anti-fogging film made of a transparent synthetic resin. More particularly, it relates to an anti-fogging film made of a synthetic resin which is effective to prevent formation of misty water droplets in the vicinity of its surface and to prevent fogging due to deposition of water droplets formed by condensation of moisture on its surface. Further, the present invention relates to an anti-fogging film suitable as a cover for e.g. a green house or tunnel.
2. Discussion of Background
Heretofore, synthetic resin films excellent in heat insulating properties and weathering resistance have been widely used as covering materials for green houses, tunnels, etc. Such synthetic resin films are advantageous in that as compared with glass plates, they are light in weight and inexpensive. In these fields of application, it is important that the covering materials have a high light transmittance and excellent anti-fogging properties. Fogging occurs mainly by condensation of moisture in air in the green house or tunnel to form water droplets on the inner surface of the covering material.
The following methods have been proposed to prevent formation of water droplets on the inner surface (the surface facing the inside of the green house or tunnel) of the covering material.
(1) A method wherein an anti-fogging agent is incorporated in the synthetic resin material, followed by filming (e.g. Japanese Examined Patent Publication No. 6845/1956)
(2) A method in which an anti-fogging agent is coated on the surface of a synthetic resin film (e.g. Japanese Examined Patent Publication No. 31508/1978)
However, by these conventional methods, it is difficult to prevent formation of a mist in the vicinity of the inner surface of the covering material when the temperature difference is substantial between the inside and outside the covering material.
As a method for preventing fogging and formation of a mist in the vicinity of the inner surface of a covering material, a method has been proposed in which an anti-fogging agent and a silicon compound, or an anti-fogging agent and a fluorine-containing compound, are used in combination, as disclosed in Japanese Unexamined Patent Publications No. 12070/1982 and No. 14648/1982.
However, when an anti-fogging agent is used in combination of a silicone compound or a fluorine-containing compounds, the anti-fogging properties may sometimes be impaired. Especially when such a material is used in a summer season or in early autumn where the temperature is high, the deterioration of the anti-fogging properties tends to be facilitated and the durability of the anti-fogging properties tends to be extremely poor.
However, the above-mentioned conventional methods also have the following drawbacks:
1 In each case, the additive is incorporated to the base resin to obtain an anti-fogging effect and an effect for preventing a decrease of the light transmittance. However, in order to avoid an undesirable phenomenon, it is necessary to select the type of the additive and to limit the amount of the additive to be incorporated. Such selection of the type of the additive and limitation of the amount of the additive require substantial work.
2 Substantial work and a long period of time are required to ascertain the presence or absence of the interaction between the additive which has already been incorporated in the base resin and the additive to be incorporated anew and to ascertain any change in the surface condition of the synthetic resin film where both additives are coexistent.
3 When the film is practically used as a covering material for a green house or tunnel, it is required to be always stable and maintain a satisfactory surface condition within a wide temperature range ranging from a temperature below the freezing point to a temperature of 70.degree. C.